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I didn't know my 5 year old bike counted as legacy
Strict loose balls are rare on modern bikes parts above BSO level, even Shimano are using caged balls on many of their hubs and "loose ball" BBs and headsets have mostly used cages for at least 30 years.
I don't expect to do this very often
Yeah, but if you're shopping anyway you might as well be prepared for next time. Balls will store successfully for decades 😀
But online bearing suppliers do. If you're setting up to be ready for legacy bike stuff, get 100 each of 1/4", 3/16", 5/32"and 1/8" from somebody like https://simplybearings.co.uk/shop/index.php?cPath=23_7240&&selection=AISI+52100+Chrome+Steel+Loose+Ball+Bearings